A) Background: Lasers
A laser consists of a pumped gain medium placed within an optical resonator. The pumped gain medium provides light amplification, and the optical resonator provides optical feedback, such that light can circulate within the optical resonator and be amplified by the gain element repeatedly. The gain medium can be pumped in various ways to provide optical amplification, including but not limited to electrical pumping and optical pumping. If the round trip loss within the optical resonator is less than the round trip gain provided by the gain element, the optical power increases on each round trip around the cavity. Since the amplification provided by the gain element decreases as the circulating optical power increases (an effect referred to as gain saturation), the steady state circulating power is the power required to make the round trip gain equal to the round trip loss. One of the elements within the optical resonator acts as the output coupler, where a certain fraction of the circulating power is emitted from the optical resonator, and constitutes the useful laser output. A partially transmitting mirror is a typical output coupler. It is frequently useful, for laser control, to provide one or more secondary output couplers in the optical resonator, where a small fraction of the circulating optical power is emitted from the secondary output coupler(s) in such a way as to be easily separated from the useful laser output.
An especially relevant laser for the present invention is an external cavity semiconductor laser. In such a laser, the gain medium is typically an electrically pumped single-mode optical waveguide. This waveguide has finite length, so it is terminated by two endfaces. Any semiconductor laser where the defining mirrors of the optical resonator are not the two endfaces of the gain medium waveguide is called an external cavity semiconductor laser, since in the course of a round trip, light leaves the gain element waveguide and reenters it. An optical beam emitted from an endface of a single-mode optical waveguide always has a characteristic amplitude and phase profile, referred to as the mode profile. This beam then propagates through the external cavity elements (i.e. everything except the gain element, usually), and returns to the gain element waveguide. It is convenient to define the “external cavity” as comprising all optical elements the beam encounters from the time it leaves the gain element waveguide to the time it returns to impinge on the waveguide endface. Frequently, the external cavity consists of a sequence of transmissive optical elements terminated by a mirror, called the return mirror, which retroreflects the optical beam back through the sequence of transmissive optical elements to impinge on the endface of the gain medium waveguide. The amplitude and phase profile of the return beam is generally not exactly the same as the mode profile, and in such cases, not all of the return beam power is launched (i.e. coupled) into the gain element waveguide. For example, if a certain power Pb impinges on the waveguide endface, only some lesser amount of power P0 is launched into the waveguide. The coupling efficiency η=P0/Pb depends on how close the return beam amplitude and phase profile is to the mode profile. We refer to loss due to launch inefficiency as mode mismatching loss.
The emission wavelength of the laser is determined by the wavelength dependence of the round trip loss and the round trip gain. If the gain medium provides amplification over a wide wavelength range, and is homogeneously broadened, then the emission wavelength of the laser will be largely determined by the wavelength at which the round trip loss in the resonator is minimized. A gain medium is homogeneneously broadened if gain saturation reduces the optical gain at all wavelengths, and is inhomogeneously broadened if gain saturation only reduces the optical gain at or near the wavelength of the saturating optical beam. Thus, the most common way to make a tunable laser is to insert optical element(s) within the cavity to create a tunable intracavity bandpass filter. Since a bandpass filter has low loss for a narrow range of optical wavelengths centered about some center wavelength λc, and high loss for wavelengths outside this range, such a filter will tune the laser emission wavelength.
B) Background: Acousto-optic Devices
While many kinds of devices have been used as tuning elements within lasers, the use of acousto-optic (AO) devices as laser tuning elements is most relevant for the present invention. Acousto-optic devices are described in textbooks such as J. Xu and R. Stroud, Acousto-Optic Devices: Principles, Design and Applications, Wiley, New York, 1992. For our purposes it is sufficient to define an AO device as any device where the AO interaction is manifested. The AO interaction is a parametric three wave mixing process where an incident optical beam and an acoustic wave interact to generate a second optical wave, with an efficiency that primarily depends on the incident optical wavelength and the acoustic frequency. We refer to the second optical wave that is generated as the first order beam, or diffracted beam; and we refer to that portion of the incident beam that is not converted to the first order beam and exits the device as the zeroth order beam, or undiffracted beam. The zeroth and first order beams may always be distinguished, since the optical frequency of the zeroth order beam is the same as the incident optical frequency, while the optical frequency of the first order beam is larger or smaller than the incident optical frequency by an amount equal to the acoustic frequency. We refer to this effect as the “frequency shift” of the first order beam. The zeroth and first order beams may also differ in other ways, such as propagating in different directions, or having different states of polarization. The acoustic wave in an AO device is typically generated by a transducer bonded to a surface of the AO device, where applied electrical power at a suitable RF frequency acts to launch an acoustic wave of the same frequency into the AO device.
Since the AO interaction is a parametric three wave mixing process, the incident optical beam is efficiently converted to the diffracted beam only for a narrow range of incident optical wavelengths, centered about some center wavelength λc at which the phase matching condition is met. The center wavelength λc of the AO interaction can be changed by changing the RF frequency applied to the AO device. Therefore, if we regard an AO device as an optical three port device, with one input and two outputs (zeroth order and first order), then transmission from input to first order output (first order transmission) gives a tunable bandpass filter, and transmission from input to zeroth order output (zeroth order transmission) gives the corresponding tunable notch filter (i.e. high loss for a narrow range of optical wavelengths centered about some center wavelength λc, and low loss for wavelengths outside this range). Although many different AO devices are shown in the literature, such as AO tunable filters (AOTF), AO deflectors and AO modulators, this general description (i.e. bandpass filter in first order transmission and notch filter in zeroth order transmission) is applicable to all AO devices.
C) Background: Acousto-optic Laser Tuning Elements
A preferred embodiment of the present invention entails the use of an AOTF in zeroth order transmission to tune an external cavity semiconductor laser. We refer to the resulting laser as a “zeroth order AOTF laser”. The novelty and nonobvious nature of the zeroth order AOTF laser is best appreciated by briefly reviewing the use of AO devices to tune lasers.
Acousto-optic devices were first used to tune dye lasers [e.g. as described in Streifer et al., Applied Physics Letters 17(8) p335 1970; Taylor et al., Applied Physics Letters 19(8) p269 1971; Hutcheson et al., IEEE Journal of Quantum Electronics, p 462 April 1974]. Subsequently, AO devices were used to tune other lasers where the gain medium has a broad bandwidth, such as fiber lasers [U.S. Pat. No. 5,189,676 Wysocki et al; U.S. Pat. No. 5,255,274 Wysocki et al; U.S. Pat. No. 5,812,567 Jeon et al], semiconductor lasers [Coquin et al., Electronics Letters 24(10) p599 1988; Koh et al., Proc. SPIE v3631 p98 1999; Zorabedian, IEEE Journal of Lightwave Technology 13(1) p62 1995], and titanium-doped sapphire lasers [U.S. Pat. No. 5,835,512 Wada et al 1998]. In addition, there are several reports in the patent literature where general methods of tuning a laser with an AO device are disclosed, such as [U.S. Pat. No. 4,118,675 Rahn et al 1978] where the AO device acts as a beam deflector and dispersive element simultaneously to force oscillation at a desired wavelength. GB 2,153,137 Hall et al. 1985, discloses the use of an AO beam deflector and separate dispersive element to tune a laser. In U.S. Pat. No. 5,724,373, Chang 1998, discloses a method for AO laser tuning where the AO interaction provides a waveguide mode conversion device, and narrowband filtering is obtained by use of this AO device in combination with appropriate polarization optics within the laser cavity.
In all of the references cited above, the AO device functions as a bandpass filter within the laser cavity, either independently, such as in the cases where the device is an AO device operating in first order transmission, or in combination with one or more intracavity optical elements, such as the combination of an AO mode converter with polarizers, or the combination of an AO beam deflector with the cavity round trip consistency condition to provide frequency selective feedback. In all the above cited references the diffracted (i.e. first order, deflected or mode converted) beam is of primary importance for the tuning mechanism, and the undiffracted (i.e. zeroth order, undeflected or non-mode converted) beam is not utilized by the tuning mechanism. All of these references teach the use of an AO device in first order transmission to tune a laser, and none teach a the use of an AO device in zeroth order transmission to tune a laser. Since first order transmission through an AO device entails a frequency shift, much of the prior art cited above is concerned with the effect of the uncompensated frequency shift on laser operation, and/or on various methods of compensating for the frequency shift so as to eliminate its effect on laser operation.
One reference teaches the use of other than the first order beam (although not the zeroth order beam) in an AO interaction to tune a laser. U.S. Pat. No. 5,384,799 to Osterwalder (1995) discloses the formation of a periodic refractive index perturbation within an AO device due to a standing acoustic wave. Osterwalder further discloses a tunable narrowband feedback mechanism into the laser cavity, based on the narrowband reflection created by the disclosed periodic refractive index perturbation. A zeroth order AOTF laser is not taught by Osterwalder.
A fundamental difference between a zeroth order AOTF laser and the laser of Osterwalder is that Osterwalder teaches the presence of a narrowband back reflection from an acousto-optic device. No such back reflection is observed from an AO device in zeroth order transmission in experiments designed specifically to manifest this effect. Furthermore, coupled mode theory, a recognized theoretical approach for analyzing acousto-optic interactions that is well known to those skilled in the art, categorically predicts no narrowband back reflection from an AO device in zeroth order transmission.
There are additional significant differences between the teachings of Osterwalder and the zeroth order AOTF laser. Osterwalder teaches the use of a standing acoustic wave to create the disclosed refractive index perturbation. In contrast, an AO device in zeroth order transmission is driven by a traveling acoustic wave. Osterwalder teaches the importance of a collinear alignment of optical and acoustic wave vectors. In contrast, an AO device in zeroth order transmission can be based on a non-collinear phase matching geometry (e.g. as disclosed in U.S. Pat. No. 5,329,397). Osterwalder discloses a configuration with a waveguide AO device. In contrast, an AO device in zeroth order transmission is not a waveguide device.
In fact, the zeroth order AOTF laser is a preferred embodiment of a novel, nonobvious, and more general laser tuning mechanism we have discovered, which is referred to herein as “spectrally dependent spatial filtering” (SDSF). Various specific embodiments of the SDSF mechanism, using different gain media and/or different tuning elements, may be preferable in certain applications. Possible gain media include, but are not limited to, Erbium-doped optical fiber and electrically pumped semiconductor gain media. Possible tuning elements include, but are not limited to, volume hologram devices, and non-AO three wave parametric interaction devices, such as electro-optic devices. Note that the AO interaction is one example of a three wave parametric interaction. Suitable control inputs for volume holograms and/or three wave parametric devices include electrical frequencies and/or voltages.
In general, tunable diffractive optical elements operating in “zeroth order” (i.e. based on the use of the undiffracted beam) are regarded as suitable tuning elements for the SDSF mechanism, and are referred to as SDSF tuning elements. In transmission through an SDSF tuning element, the total power in the optical beam may decrease, due to attenuation within the tuning element, and the amplitude and/or phase profile of the beam may be changed, which is herein referred to as “distortion”. This distortion and attenuation will depend on the transmitted wavelength. Note that an SDSF tuning element as defined here is not obviously a suitable tuning element for a laser, since an SDSF tuning element typically acts as a notch filter in isolation, and laser tuning requires a bandpass filter.